There are several methods for coating solid with one of more chemical substances. The carrier particles can be added to a fluidised bed and the coating solution of a chemical substance in a solvent is then added to the fluidised bed and the solvent evaporated. This method gives good coating results, but the amount of air needed to form the fluidised bed is such that the evaporated solvent (which in most cases is an organic solvent) contaminates a large volume of air, which can not without further treatment be vented in the open atmosphere. Therefore the installations for fluidised bed coating are quite large and expensive. Moreover, in a fluidised bed the carrier particles are strongly agitated and many collisions occur which damage the coating around the carrier particles.
In an other method, the carrier particles are mixed (dispersed) in a solution of the chemical compound or compounds that are to be applied on the surface of the carrier particles and the particles are the spray-dried. Again this method gives good coating results, but the installation needed for spray drying is expensive.
In several documents it is disclosed to bring a solution of coating compounds in a low temperature boiling solvent in contact with the carrier particles to be coated in a vessel while mechanically stirring and to evaporate the solvent afterwards or simultaneously. Documents disclosing such methods are, e.g., U.S. Pat. No. 3,507,686, U.S. Pat. No. 3,947,271, U.S. Pat. No. 5,102,769, U.S. Pat. No. 5,340,677 and GB-A-2,014,876.
When coating carrier particles for use in electrostatic imaging the coating has to very homogenous over the surface of the particles to be coated. In electrostatographic imaging the carrier particles are mixed with toner particles (and with other ingredients) to form a developer. When the developer is to be used in magnetic brush development, the carrier particles are magnetic, when the developer is used in cascade development, the carrier particles can be coated glass beads. In any case the rubbing of the carrier particles and the toner particles induce a tribo-electric charge in the toner particles and the nature of the coating on the carrier particles determines, together with the toner ingredients, the polarity of the charge on the tone particles as well as the amount of the charge. When the coating of the surface of the carrier particles is not even and has interruptions, then problems in charging the toner particles can occur. Thus in coating carrier particles it is of utmost important to have an even, closed coating on the surface of the particles. In the prior art methods, frequently some of the particles adhere, during the process to the wall of container of the fluidised bed or spray drying apparatus, and these particles are only coated from 1 side.
Very homogenous coating of particles are not only desirable for carrier particles to be used in an electrostatic developer, but also for other particles, e.g., particles that are used in absorption chromatography.
Thus it is still desirable to have a simple, inexpensive, environmentally sound method for coating particles with an homogenous surface layer.